1. Field of the Invention
This invention concerns a method and a device for performing sample injection under controlled conditions of the temperature profile into gas chromatographic columns, in order to avoid unwanted volatilizations of the sample compounds outside the injection syringe needle, in case of injection by means of "on column" type injectors, for example of the type described by Grob & Grob (Journal of Chromatography 151, page 311 (1978)), and also to ensure efficient "trapping" effect and suitable vaporization of the sample compounds entrapped in the initial part of the chromatographic column. The mentioned trapping effect may be obtained, according to the invention, not only by means of the above mentioned injectors, but also using other types of injectors or sample injection systems (for instance, split/split-less injection) used for capillary columns or packed columns. As it was proved in practice, in case of injectors of the above mentioned "on column" type, it is important that no unwanted volatilization of the sample compounds occurs during the sample injection into the column; these volatilizations may occur in case the needle temperature, and therefore the sample temperature, exceeds certain limits during the injection or at the column inlet.
2. Description of the Prior Art
As it is well known practically all injection systems for liquid samples, presently used in gas chromatography cannot carry out suitable sample injection without an additional "trapping" operation. This is due to the fact that, frequently, the sample volumes exceed the maximum values required and accepted to assure the chromatographic system efficiency.
Therefore, part of the gas chromatographic capillary column initial section is used as cold trap, causing condensation and/or solution of the sample compounds. Up to now, this is practically achieved by gradually freeing the components by means of a suitable temperature program of the oven where the column is positioned.
This trapping effect is useful when a split-less type injection is used, but it very often results advantageous even during the usual splitter-type injection. When diluted samples are injected using the split-less injection system, part of the solvent is condensed together with the sample compounds, thus increasing the trapping capacity of the column initial section part and usually determining a good definition of the peaks indicating the sample compounds during analysis. However, this solvent effect has sometimes undesired consequences. If an "on column" injection system is used, as previously indicated, the samples are directly injected as diluted solutions at the inlet part of the capillary column. In this case, too, the compounds must be entrapped in a zone of the column near the injection port and the operation must be carried out in such a way that most of the solvent is evaporated in a sufficiently slow way in order to avoid creation of a shock wave due to sudden vaporization, but anyway sufficiently rapidly to avoid formation of a liquid cap moving downwards through the column.
The presently used trapping method requires cooling of the whole column and therefore of the whole oven where the column is placed in order to obtain the above mentioned effects of trap and solvent. The method proposed according to this invention, on the contrary, enables to obtain the effects of both trap and solvent by controlling the temperature of a more or less long section of the column initial part, without altering the oven temperature conditions. Moreover, the above mentioned method versus the known technique also has the advantages of allowing to bring the column initial part to any temperature, for instance -80.degree. C., to save power and time in comparison with conventional procedure and to perform quick temperature change in the column initial part, with limited power consumption, thanks to small thermal inertia of the whole system.